For power plants of the kind referred to here, no well-tried technique exists for superheating of steam between two turbine stages or between a high pressure turbine and a low pressure turbine. A choice between two principles is possible:
1. A separate nest of boiler tubes for intermediate superheating of steam is located in the common bed vessel. This embodiment gives insufficient possibilities of obtaining optimum steam data. The superheating tube nest can be dimensioned so as to obtain optimum steam data at full load. The tubes in the tube nest can be distributed in horizontal layers in such a way that the tube area above the bed and in the bed, respectively, is of such a magnitude as to obtain as suitable a superheating as possible at partial load. However, the dimensioning and the distribution of the tubes make it impossible to obtain optimum intermediate superheating of the steam. This applies particularly to the case of partial load. Maladjustment between steam flow and tube area means that it is necessary either to inject water to prevent an impermissible increase in temperature in the tube nest, or that it must be accepted that no optimum superheating is obtained. In both cases, the efficiency of the power plant is reduced.
2. A tube nest for intermediate superheating of steam is located in a separate bed vessel. This embodiment makes it possible in the desired way separately to control the intermediate superheating and obtain optimum steam data for different turbine stages under all operating conditions. The plant is complicated by the fact that each one of the beds has to be provided with complete control systems for an air supply, fuel supply and bed depth control, for example a doubling of the control systems.